Production of human stem cell-into-animal chimeras
In this case human stem cells are injected into an animal at the fetal stage or older. This method is used primarily for proof-of-principle for establishing certain properties of the human stem cells (i.e. differentiation potential, engraftment potential, or their potential therapeutic efficacy for specific diseases or injuries). There are many examples in the literature of the production of this type of chimera for experimental purposes. These include the production of a human hematopoietic system after bone marrow transplantation in the fetal sheep and the investigation of the neurogenic potential of human neural stem cells transplanted into the fetal mouse brain. A concern that has been raised about the production of this type of chimera is one regarding the possible induction of "human qualities" in the recipient animal. The two primary concerns are higher level neurological function and the appearance of recognizable human features. For most, the more worrisome of these two possibilities is the development of human-like intelligence, consciousness, or emotion. Fortunately, this is highly unlikely. Many of the critical developmental signals for the formation of the human brain will have already occurred or signaling necessary for a developing human nervous system will be absent in the developing animal nervous system. Introduction of human stem cells at the blastocyst or very early embryonic stage, however, may give entirely different results; therefore, this type of transplantation is considered of high ethical concern and is dealt with below.
Use of hESCs that are ethically derived, as described above, is currently permissible, even with Federal funds, if the cells were derived before August 9, 2001.
This area includes the xenobiotic transplantation of animal stem cells into humans. Although this has not yet been done, or even proposed to our knowledge, xenobiotic organ transplantation has. It is not unreasonable to imagine that a clinically useful stem cell population might more easily be differentiated from animal rather than human source material.
The primary long-term goal for stem cell research is to develop strategies for preventions, treatments, and cures that can be used in humans. Both the necessary clinical trials and the resulting clinical applications raise issues that are distinct from the basic research now underway. The possible experimental approaches are extensive, but it is noteworthy that a variety of clinical applications are already widely, even if not universally, accepted: (1) transplantation of tissue-specific (e.g. bone marrow, cord blood, peripheral blood) stem cells to treat hematologic or metabolic diseases; (2) IVF for the purpose of reproduction; (3) donation of eggs for reproduction; and (4) PGD for reproductive screening.
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